How to make widget fill entire mainwindow

[Morea]

How can I make a Tab widget fill the entire mainwindow, and keep the Tab widget filling the mainwindow regardless of how I resize the mainwindow?
I design the layout in the designer.

[Junior]

Morea,

In designer open the object Inspector dialog and click on centralwidget under the MainWindow. This will load the properties menu for the central widget and from there you can adjust the layout parameters of central widget.

[def]

I've got a similar problem to this. If I type code myself I can do this:
#include <QtGui/QApplication>
#include <QtGui/QPlainTextEdit>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QPlainTextEdit text;
text.show();
return a.exec();
}
and I get a text editor that fills my window and resizes with it. I've been trying to do the same thing with the Desiger and the text editor is always a fixed size which doesn't change when I resize the window.
> In designer open the object Inspector dialog and click on centralwidget under the MainWindow.
I'm not absolutely sure which thing on the screen is the object Inspector.
> This will load the properties menu for the central widget and from there you can adjust the layout parameters of central widget.
I've tried adjusting various properties of this widget and nothing makes any difference. Which property should be adjusted? and what value does it need to be set to.